The invention generally relates to medical devices and procedures. The invention more particularly concerns a polymeric construct having interlaced and interlocked fibers and products formed from the fibrous polymer.
To better treat our aging population, physicians are looking for new and better products and methods to enhance the body's own mechanism to produce rapid healing of musculoskeletal injuries and degenerative diseases. Treatment of these defects has traditionally relied upon the natural ability of these types of tissue to repair themselves. In many instances the body is unable to repair such defects in a reasonable time, if at all. Advances in biomaterials has allowed for the creation of devices to facilitate wound healing in both bone and soft tissues defects and injuries. Such devices are used in tissue regeneration as tissue (e.g., bone) graft scaffolds, for use in trauma and spinal applications, and for the delivery of drugs and growth factors.
Bone and soft tissue repair is necessary to treat a variety of medical (e.g., orthopedic) conditions. For example, when hard tissue such as bone is damaged as a result of disease or injury, it is often necessary to provide an implant or graft to augment the damaged bone during the healing process to prevent further damage and stimulate repair. Such implants may take many forms (e.g., plugs, putties, rods, dowels, wedges, screws, plates, etc.), which are placed into the tissue. Typically, such implants can be rigid, flexible, deformable, or flowable and can be prepared in a variety of shapes and sizes. For non-rigid structural repair materials (e.g., putties and pastes) to be conveniently used, they must be capable of being formed into a variety of complex shapes to fit the contours of the repair site. An accurately configured implant that substantially fills the defect site will enhance the integration of natural bone and tissue to provide better healing over time. The prior art discloses medical implants that comprise, at least partly, collagen (to be discussed later).
Collagen is the most abundant protein found in the body. The unique chemistry of collagen makes it an ideal polymer for structural and hemostatic applications in both clinical and diagnostic settings. Collagen, like all proteins, is comprised of amino acids linked covalently through peptide or amide linkages. The sequence of the amino acids, or the primary structure, outlines the three-dimensional structure of the protein, which in turn dictates the function, and properties of the molecule. Collagen is composed of three peptide chains associated in a triple helical orientation. These triple helices associate to form fibrils, which ultimately make up connective tissue and other structural members.
Collagen has been used in a number of applications in the art. For example, one application is for use in hemostatic devices for the stoppage of bleeding, such as is described in U.S. Pat. Nos. 5,310,407 (Casale) and 4,890,612 (Kensey). However, neither teaches the use of native insoluble fibrous collagen. In U.S. Pat. No. 5,425,769, Snyders, Jr. discloses a biocompatible and bioresorbable bone substitute with physical and chemical properties similar to bone, consisting of reconstituted fibrillar collagen within a calcium sulfate di-hydrate matrix. The ratios of calcium sulfate and collagen are adjusted for each application and the bone substitute is molded in situ to form a solid phase. Snyders Jr. discloses an implant that remains malleable only for a brief period, as the combination of fibrillar collagen and calcium sulfate di-hydrate matrix forms a hard composition. Furthermore, the collagen as described in the '769 patent is neither interlocked, nor interlaced, relying on the calcium sulfate to lend structural integrity.
The polymer utilized for the implant may be combined in application with a biologically active agent to enhance the tissue healing response or enhance the mechanical properties of the implant (e.g., U.S. Pat. No. 4,776,890 (Chu)). Chu discloses a process for creating matrix of collagen containing mineral particles, such that when wetted, the matrix is malleable and retains its integrity. The matrix as claimed by Chu incorporates up to 10% of the mass as collagen, and relies on the physical characteristic of the particles comprising the bulk of the matrix to lend the integrity, and upon exposure to fluids, would lead to dissociation of the material unless a cross-linking step is performed. However, this cross-linking process is disfavored by Chu, as it would discourage bone tissue ingrowth.
Huc et al. (U.S. Pat. No. 5,331,092) describes a process for preparing medical pads by grinding collagen, acidifying with acetic acid, homogenizing, molding and freeze-drying. The pad formed would readily fall apart upon exposure to aqueous fluids and thus requires cross-linking. The cross-linked pads hold together but have limited mechanical strength limiting their usefulness to hemostatic pads.
Nigam (U.S. Pat. No. 4,948,540) described a process for preparing a collagen dressing material by creating a slurry comprised of an acid solubilized collagen and a non-solubilized natively cross-linked collagen. The resultant slurry was molded, and freeze-dried into a pad. The pad did not have sufficient mechanical properties due to its excessive porosity and thus was compressed at a pressure of 15,000–30,000 psi and optionally cross-linked. To improve strength due to lack of fiber-to-fiber interaction, the device is compressed without interlacing of the individual fibers. The compression serves to compress in only one dimension, placing the fibers in close proximity in one orientation, rather than interlacing the fibers.
Li (U.S. Pat. No. 5,206,028) described a process for preparing a dense collagen membrane by first freeze-drying a collagen dispersion of random fibers to form a sponge. This sponge was then humidified, compressed and subjected to chemical cross-linking. The resultant sponge was strong, having randomly entangled masses of fibers going in all directions. This device as described by Li lacks interlacing of the insoluble collagen as the aqueous dispersion is lyophilized without first interlacing the insoluble components.
Li (U.S. Pat. No. 6,391,333) described a process wherein sheets of oriented biopolymeric fibers are formed into sheets by capturing them on a spinning mandrill that was rotated in a fibrous collagen slurry. The fibers were then compressed to force them closer together so they could be dried, preferably while in contact with a gluing agent. The sheet was then cut from the mandrill, inverted and cross-linked to form a sheet. Additional sheets could be individually stacked on top of each other to create thicker devices with greater mechanical strength. The device as constructed has fibers substantially aligned in parallel planes, and lacks equiaxial interlacing.
In PCT application WO 98/35653, Damien describes a process for preparing an implantable collagen putty material by acidifying a collagen solution to a pH of between 3.0 to 6.0. This produces a non-fibrous dough like material that can be used to suspend graft material. At higher pH, the collagen precipitates out, becoming crumbly with a consistency of wet sand.
It is well known to utilize a centrifuge or filtration press as a part of a rinsing procedure, or a ‘wash step’ to remove insoluble components contained within the solution. Nishihara (U.S. Pat. No. 3,034,852) describes a process to solubilize previously insoluble collagen fibers without denaturation of the protein structure by using hydrolytic enzymes. In the examples, the author describes separation of the fibers from the wash solution by centrifugation or filtration press methods, the fibers are then brought back into solution. Additionally, the fibers, which are separated using this method are reconstituted fibers which tend to be small in size.
Highberger, et. al. (U.S. Pat. No. 2,934,446 and U.S. Pat. No. 2,934,447) describe a method, as well as, the physical preparation of collagen fiber masses to form leather-like sheets from hide scraps unusable in the traditional leather making process. This psuedo-leather may support small colonies of cells but would be unsuitable for tissue ingrowth. The method of concentration used is a precipitation technique, which creates a fiber dispersion. This slurry/dispersion as described included random clumps of undispersed or entangled fibers. Highberger combines a unique fiber that coacts with a high dissolved solids content collagen solution to form well knit, or leather like sheet. In the '447 patent, Highberger further refines the process of the '446 patent by incorporating a kneading step, which works the dough material to make the product free from lumps, the kneading necessarily disrupts any interlacing or interlocking fibers prior to precipitating the solubilized collagen.